armv7m: Re-enable unaligned accesses

The Keil RTX libraries are compiled with unaligned accesses enabled, and
so it is possible for RTX to trigger unaligned access violations with
otherwise valid code. Unaligned accesses have been re-enabled to reduce
the risk of encountering avoidable exceptions.

Change-Id: I6b5867dda919db65033bfe6d889da0fd69ab4482
Signed-off-by: Chris Kay <chris.kay@arm.com>